The cadmium sulfide (CdS) or light dependent resistor (LDR) whose resistance is inversly dependent on the amount of ambient light intensity, is known by many names including the photo resistor, photoresistor, photoconductor, photoconductive cell, or simply the photocell.

Its resistance decreases from millions of ohms in darkness to a few hundred ohms in bright light.  Each application requires an appropriate circuit and physical setup, and some calibration may be required for the exact lighting scenario encountered. Able to detect large or small fluctuations in light levels, it can distinguish between one or two light bulbs, direct sunlight, total darkness, or anything in between.

A typical structure for a photoresistor uses an active semiconductor layer that is deposited on an insulating substrate. The semiconductor is normally lightly doped to enable it to have the required level of conductivity. Contacts are then placed either side of the exposed area.

The photo-resistor, CdS, or LDR finds many uses as a low cost photo sensitive element and was used for many years in photographic light meters as well as in other applications such as smoke, flame and burglar detectors, card readers and lighting controls for street lamps.

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